This invention is particularly concerned with the detection of frequencies which are within a predetermined range. A common prior art method of detecting frequencies is to use a phase locked loop but there are a number of drawbacks to the utilization of such loops. One drawback is that rather high current consumption is required and another is that the signal being detected must be present for many cycles before the circuitry responds. Furthermore, the response time is lengthened considerably as the range of frequencies to be detected is narrowed about the center frequency of the phase locked loop.